This invention relates to a nonaqueous electrolyte secondary battery, and more particularly to cathode active material for a nonaqueous electrolyte secondary battery and the manufacturing method thereof.
Recently, consumer electronic devices are rapidly becoming more portable and cordless, and as the power supply for driving these electronic devices, there is a high demand for secondary batteries that are compact, lightweight and have a high energy density. From this aspect, there is large expectation and a rush for development for a nonaqueous electrolyte secondary battery, and particularly a lithium secondary battery having high energy density.
A nonaqueous electrolyte secondary battery comprises a cathode, an anode and a separator that is located between them, where a microporous membrane made mainly of polyolefin is used for the separator. An aprotic organic solvent, in which a lithium salt such as LiBF4, LIPF6 or the like is dissolved, is used as the nonaqueous electrolyte.
Recently, a battery that contains a lithium composite oxide as a cathode active material, and contains a carbon material, silicon compound, tin compound or the like as the anode material is attracting much attention as a lithium secondary battery having high energy density. A lithium oxide, for example, lithium cobalt oxide (LiCoO2), which has a high electric potential with respect to lithium, is very safe and can be combined relatively easily, is being used as the lithium composite oxide.
Also, in order to avoid the problem of Cobalt resources, and from the aspect of even higher capacity, much testing is being performed for using lithium nickel oxide (LiNiO2). There are abundant resources of nickel, and besides being able to be manufactured at low cost, lithium nickel oxide is also very suitable for high capacity. However, the crystal thermal stability of lithium nickel oxide having high capacity is low, and there is a problem with its cycle characteristics and high temperature storage characteristics. Therefore, materials such as the following have been proposed.
In Japanese Patent Application Publication No. H5-242891, doping lithium nickel oxide with cobalt and aluminum is disclosed from the aspect of improving the thermal stability of the lithium nickel oxide. However, in regards to the thermal stability, a certain amount of improvement is seen, however, in regards to the cycle characteristics and high temperature storage characteristics, adequate characteristics are not obtained.
Also, in H. Arai et al., J. of Electrochem. Soc., 144 (1997) 3117 a method is disclosed of improving the thermal stability by substituting nickel with cobalt, manganese or titanium. However, in this method of substitution, there is a problem in that the capacity of the secondary battery is decreased.
On the other hand, in H. J. Kweon et al., Electrochem. And Solid-State lett., 3 (2000) 128, a method is disclosed for improving the thermal stability by coating the surface of lithium nickel cobalt oxide with magnesium oxide. However, magnesium oxide is not an active material, so that coating reduces the charge and discharge capacity of the cathode active material.
Also, in Japanese Patent Application Publication No. H9-55210 a coating process is disclosed in which the surface of composite oxide particles that are represented by LiNixMyO2 (where M represents at least one kind of element that is selected from among Al, Ni, Co, Cr, Ti, Zn, P and B, and 0<x≦1, 0≦y≦1) is covered by a compound that contains cobalt, aluminum or manganese. However, when the surface is covered with a compound, there is a concern that the specific surface area of the cathode active material is decreased, the contact between the active material and the electrolyte is reduced, the dispersion of the lithium is hindered, and the battery capacity is reduced, also the processing time of the disclosed method of manufacture is long and cannot be used as an industrial method. Moreover, only an example of using metallic alkoxide as the compound was given, however, there is a problem in that after processing there is residual carbon, and it combines with lithium to generate lithium carbonate, and as an impurity it causes internal resistance in the battery.
Furthermore, in Japanese Patent Application Publication No. 2002-231227, a method is disclosed in which the surface of a layered oxide for the cathode of a lithium secondary battery is coated with a lithium transition metal oxide. In this technique, the raw material solution for surface processing is adjusted to a pH of 5 to 9 and a solution density of 0.1 to 2 moles and a layered oxide is added, and using the Gel-Sol method, lithium transition metal oxide is adhered to that layered oxide. After coating, by performing heat treatment of the layered oxide at 500 to 850° C. for 3 to 48 hours, a cathode active material that is made from a layered oxide whose surface is coated with a lithium transition metal oxide is obtained. However, the manufacturing method has problems in that there is a large number of processes, the industrial cost is high, the viscosity of the slurry is high, a surplus of impurities remains on the surface of the cathode active material, dispersion of lithium is hindered, and the charging and discharging capacity of the cathode active material is decreased. Also, the raw material solution for surface treatment is adjusted to a pH of 5 to 9, however, there is a possibility that lithium will elute out from the layered oxide and there will be insufficient lithium in the layered structure, so there is concern that the battery capacity will decrease. Furthermore, there are examples of using LiMn2−xM1xO4, LiCo1−xAlxO2, LiNi1−xAlxO2, LiNi1−x−yCoxAlyO2, LiNi1−x−y−zCoxM1yM2zO2 (where M1 and M2 are Al, Ni, Co, Fe, Mn, V, Cr, Cu, Ti, W, Ta, Mg or Mo, and 0≦x<0.5, 0≦y<0.5, 0≦z<0.5), however, even though these are oxides having movable lithium ions, when using the cathode active material, the charging and discharging capacity decreases about 8%. Therefore, in the case of this proposal as well, it is difficult to say that the both the problems of high capacity and safety are satisfied.
As described above, in regards to improving a cathode active material that uses lithium nickel oxide, it is a fact that there are no effective measures that provide both thermal stability and cycle characteristics without losing the advantage of high capacity that lithium nickel oxide intrinsically has.
[Patent Document 1]
    Japanese Patent Application Publication No. H5-242891[Patent Document 2]    Japanese Patent Application Publication No. 2006-201779[Patent Document 3]    Japanese Patent Application Publication No. H9-55210[Patent Document 4]    Japanese Patent Application Publication No. 2002-231227[Non-Patent Document]    H. Arai et al., J. of Electrochem. Soc., 144 (1997) 3117[Non-Patent Document]    H. J. Kweon et al., Electrochem. And Solid-State lett., 3 (2000) 128